I'd recommend you read my thread about this build:
https://forums.servethehome.com/index.php?threads/tyan-s8030gm2ne.28914/. The S8030 motherboard is a server oriented EPYC motherboard. It won't take Threadripper and AMD 39xx CPUs, only the AMD EPYC 7XX1 and 7XX2 CPUs. And these CPUs support eight channel memory and large amounts of RAM. But it has to be ECC RDIMM/LRDIMM, neither the board nor the CPUs accept unbuffered ECC DIMMs
If you already have the 3900x, you're better off with something like an AsrockRack X570D4I-2T. Newegg has it for $430. The 3900X, 3950x and Threadrippers will be faster than their corresponding core-count equivalent EPYC chips. What they wont have is support and validation when you're installing ESXi for example. If that's important or the number of PCIe slots are important - EPYC is the way to go. If those things don't matter to your use case, there's no real reason to get EPYC
Because the EPYC motherboards have 5 to 7 PCIe 4.0 slots, it is easy to add a dual 10G network card. One can get Intel X550-T2 cards off eBay for $150 to $200 and there's loads of other NICs (Mellanox, Solarflare, Broadcom, Chelsio) that will do 10G and are under $100 or even $50, it's all down to which OS you're deploying and what features you want support for